The present invention relates to an integrated circuit semiconductor device having an improved isolation region for isolating circuit elements formed in a semiconductor substrate, and more particularly to an improved isolation of insulated-gate type field effect transistors (hereinafter abbreviated as IGFET's) of the N-channel type formed in a P-type area of a semiconductor substrate.
In an integrated circuit semiconductor device, circuit elements are isolated each other by a thick field silicon oxide layer partly embedded in a semiconductor substrate of one conductivity type with a channel stopper region of one conductivity having a higher impurity concentration than that of the substrate being provided under the bottom of the embedded field silicon layer. The isolation region consisting of the field silicon oxide layer and the channel stopper region must realize a high threshold voltage in the field area. For that reason, it is desirable to make the field oxide layer as thick as possible and impurity concentration of the channel stopper region as high as possible. However, the thickness of the field oxide layer is restricted to some extent because a too thick field oxide layer sacrifices the required flatness of the surface of the device and produces crystal defects in the active region, that is, circuit element forming region. On the other hand, the impurity concentration of the channel stopper region is also restricted, because an impurity for forming the channel stopper region must be introduced into the substrate prior to a thermal oxidation process for forming the thick field oxide layer. The thermal oxidation process is conducted under a high temperature for a long time and therefore, the impurity concentration of the channel stopper region cannot be increased to a level higher than a certain limit. Moreover, even if the impurity concentration of the channel stopper region could be increased, the junction breakdown voltage decreases between the channel stopper region and one region of the circuit element such as a source or drain region of the IGFET which is in contact with the channel stopper region. Thus, the upper limit of the impurity concentration of the channel stopper region under the partially embedded field oxide layer was 10.sup.18 atoms/cm.sup.3.
Further, when the device of the above-mentioned structure is irradiated with ionizing radiations (.gamma. rays, .alpha.-rays or electron rays), positive holes in the electron-positive hole pairs formed in the silicon oxide layer migrate into the interface between the silicon substrate and the silicon oxide layer, and are trapped near the interface. Therefore, a fixed electrical charge of positive polarity accumulates in the silicon oxide layer. In the case of a P-type silicon substrate, the electrical field is established in a direction inverse to the surface of the silicon substrate and therefore, the leakage current is apt to flow by the irradiation of the ionizing radiations. The leakage current increases with the increase of the irradiation. Further, the ionized positive holes are produced more in a thick silicon oxide layer than in a thin silicon oxide layer and therefore, the leakage current due to the ionizing radiations are increased under the thick silicon oxide layer at the isolation region. Therefore, if the effect by the irradiation of the ionizing radiations is to be considered, the field insulating layer on the P-type substrate area to isolate N-type regions must be thin.